Carbon/carbon material denotes a porous material having only carbon left therein due to cycles of carbonization and impregnation. The porous portions locally have a density difference. The combustion test results show an inconsistent erosion rate in spite of using carbon/carbon materials having similar densities with respect to an entire volume, which is caused directly due to the local density difference of the carbon/carbon materials.
The local density of the carbon/carbon material can be physically computed by cutting, dissection and the like, but such mechanisms come with destruction of the carbon/carbon material. Consequently, supplementary data is merely obtained at the beginning stage of developing the carbon/carbon materials. Furthermore, if densities of those physically destructed carbon/carbon materials are computed, excessive time and cost are required. To overcome such drawbacks, a nondestructive testing for evaluating density more quickly and accurately has been introduced.
Examples of the method for nondestructively evaluating density include an ultrasonic density measurement, a density measurement using x-ray radiography, a computed tomography and the like. However, even if adapting one of such techniques, it is difficult to quantitatively measure a physical density. Studies on the density measurement have actively been conducted beginning with a bone mineral density measurement for diagnosing osteoporosis patients in a medical industry field. Nevertheless, density measurement still relies on experiences of engineers and causes large deviation between engineers, and different results are derived by the same engineer.
The ultrasonic density measurement is a method for obtaining density by using variations of delivery speed of ultrasonic waves or modulus of elasticity depending on media. However, this method has a disadvantage of low reproducibility and accuracy.
There are many methods proposed for measuring bone mineral density using X-ray images, but the difference of X-ray attenuation cannot be obviously specified with only one X-ray image, thereby generating considerable measurement errors. To overcome this, a study has been conducted on a technique for measuring thickness of a medium and a density distribution thereof using a metal step-wedge having plural steps; however, such technique can merely provide qualitative analysis results for density other than quantitative results.
A technique of measuring the density variation and density distribution of bone tissues using annihilation radiation has been disclosed in Densitometer for Determining the Density Distribution and Variation of Density of an Object filed by Bolotin in U.S. patent application (U.S. Pat. No. 5,774,520) in 1998. A positron emission source Na-22 is present at the center of X-ray detectors. One detector is designed to directly detect 511-KeV annihilation radiation generated from Na-22 and another detector measures the density of a target object placed between the Na-22 and the another detector by virtue of reaction between the 511-KeV annihilation radiation and the object. This technique uses monochromatic gamma rays so as to have an advantage of non-occurrence of beam hardening and also does not measure scattered radiation so as to have an advantage of acquisition of clear tomographic images. However, the density difference has merely been discriminated by measuring linear attenuation coefficients of bones, fat, muscles and the like and no quantitative density value has been given. Also, the use of 511-KeV annihilation radiation has limitation to a transmission capability through an object and the use of radioactive isotope is always linked to the problem of the radioactive safety supervision.
An error-correction technique, which permits very accurate imaging of the contours of the bones in vivo by producing a standard test block corresponding to the density and shape to be measured, has been introduced in X-ray Tomography Phantoms, Method and System filed by Roberton in U.S. patent application (U.S. Pat. No. 4,873,707) in 1989. The shape correction has been realized by varying internal and external diameters of the standard test block, and an empty space is present within the standard test block to be filled with a material. Such use of the standard test block can be useful for fabrication of orthopedic prostheses with a specific bone structure and correction of the bone structure; however, no quantitative density value has been provided.
A technique of measuring geometrical width and density of an object to be imaged by using density contours measured with computed tomographic images has been developed. This technique is disclosed in Method and Apparatus for Evaluation of Structural Width Tomography filed by Hangarter in U.S. patent application (U.S. Pat. No. 5,673,303) in 1997. The existing tomography technique could not measure the accurate width and density with respect to a structure having a width below a minimum resolution value of image; however, this technique has measured the width of a specific structure, the width smaller than the minimum resolution value, by measuring a full width at half maximum (FWHM) based upon a maximum density value. The technique is capable of measuring thickness and density of cortex bones; however, the density measurement has been made simply by the sum of densities within the full width at half maximum, and thereby the qualitative results are merely provided.
The quantitative density profile of carbon/carbon material by employing a computing tomography has been disclosed in Density Profile Evaluation of Needle-punched Carbon/Carbon Composites Nozzle Throat by the Computed Tomography, introduced in the first paper, vol. 10, in the journal of the Korean Society of Propulsion Engineers Transaction in 2006. In this technique, for evaluating the density profiles, the density resolution test block and standard density test block has been measured so as to measure the density profile by using standard density materials of the standard density test block.
However, this technique has a disadvantage that the local density cannot directly be measured because the correlation between the density and the linear attenuation coefficient of different types of materials, such as distilled water, NaCl water solution, magnesium and PVDF, which are inserted in inner holes made of acryl, is different from the correlation between the density and the linear attenuation coefficient of a carbon group. To overcome such disadvantage, it has been assumed that the difference of the linear attenuation coefficients with respect to the distilled water and NaCl water solution obtained from the standard density test block is to be equally applied to the carbon/carbon material, and the entire density of the carbon/carbon material has been measured, followed by indirect measurement of the local density using the difference of the linear attenuation coefficients between the distilled water and the NaCl water solution. Thus, this technique must perform complicated processes of correcting each slice image to evaluate the local density.
In addition, the standard density test block produced in the technique uses the acryl-based material other than the carbon/carbon material, which disables a direct correction of beam hardening, resulting in a disadvantage of correcting and checking the beam hardening by using the cylindrical carbon/carbon material.